1. Field of the Invention
The present invention relates to a brake control system for automotive vehicles, and specifically to a computer-controlled braking system with a skid control unit (an ABS unit) acting to prevent a wheel lock-up condition during deceleration and provide maximum effective braking, and a traction and vehicle dynamics control unit acting to suppress a so-called acceleration slip which may often occur when rapidly accelerating or being capable of actively producing a proper yawing moment necessary for improving a vehicle stability or a vehicle dynamics (e.g., a cornering stability or steer characteristics) by virtue of a computer-controlled wheel-brake cylinder pressure.
2. Description of the Prior Art
In recent years, there have been proposed and developed various computer-controlled braking systems with both a skid control unit (of ten called an "ABS unit" or an "ABS hydraulic unit") and a traction and vehicle dynamics control unit (substantially corresponding to a "traction-and-yaw control hydraulic unit"). In such a conventional computer-controlled braking system having both an ABS unit and a traction-and-yaw control unit, the ABS unit usually includes a plurality of fluid-pressure control valves and a return pump, often called an "ABS pump", required for a skid control. On the other hand, the traction-and-yaw control unit generally includes a brake-fluid pressure apply pump (or a brake-fluid pressure supply pump) serving to rise a wheel-brake cylinder pressure during the traction control or during the vehicle dynamics control (yaw control). On more later-model anti-skid braking systems with an ABS unit and a traction-and-yaw control hydraulic unit, a brake-fluid pressure apply piston is further disposed between the brake-fluid pressure apply pump and each brake circuit of a dual-brake system for the purpose of indirectly supplying the brake-fluid pressure via the pressure apply piston unit. One such computer-controlled braking system has been disclosed in an international application No. PCT/DE93/00802, filed Sep. 4, 1993 (PCT publication No. WO 94/07720, published Apr. 14, 1994), corresponding to Japanese patent publication No. 7-501506, published Feb. 16, 1995.
The braking system with an ABS device, disclosed in the international application No. PCT/DE93/00802, is applied to an automotive vehicle with a dual-brake system in which a hydraulic brake system is split into two sections or two hydraulic circuits, for example a front section and a rear section, respectively connected to two associated wheel-brake cylinders. The ABS hydraulic unit is designed to join, at a confluent point or a connection port, two kinds of brake-fluid pressures, namely a master-cylinder pressure generated from an outlet/inlet port of the master-cylinder pressure and a discharge pressure of working fluid discharged from the return pump of the ABS hydraulic unit, and then supplies a controlled hydraulic pressure, being properly regulated or controlled by a hydraulic pressure control valve unit (which is component parts of the ABS unit and typically comprises a plurality of electromagnetic solenoid valves such as two-port two-position directional control valves), into a wheel-brake cylinder which is subjected to a skid control. The brake-fluid pressure apply piston unit is arranged in a brake line close to the tandem master cylinder rather than the ABS unit. The pressure apply piston unit slidably accommodates therein a partitioning piston through which an internal space defined in the cylinder portion of the pressure apply piston unit is partitioned into a primary chamber communicating the outlet of a brake-fluid pressure supply pump (an auxiliary pump) and a secondary chamber being connected to the ABS unit. A directional control valve is also disposed in the associated brake circuit between the ABS unit and the secondary chamber of the pressure apply piston, in such a manner as to maintain the associated brake circuit at a fully-open state during its de-activated state, and to serve, during its activated state, as a pressure-differential sensitive external-pilot operating type directional control valve that functions to permit fluid flow from the ABS unit to the secondary chamber when the pressure differential between inlet and outlet ports of the directional control valve is above a predetermined value, and to block fluid flow from the ABS unit to the secondary chamber when the pressure differential is below the predetermined value. A two-port two-position directional control valve is fluidly disposed between the secondary chamber and the suction port of the return pump (the main pump) of the ABS unit for establishing fluid communication therebetween under its activated state (fully opened), and for blocking the fluid communication under its de-activated state (fully closed). In the pressure apply piston unit, an intermediate small-diameter section of the partitioning piston defines a pressure chamber in conjunction with the inner cylindrical peripheral surface of the cylinder portion. The pressure chamber communicates with one outlet/inlet port of the dual master cylinder. Also provided in the secondary chamber is a check valve which is designed to permit fluid communication between the pressure chamber and the secondary chamber under a condition in which a volumetric capacity of the primary chamber is reduced to the minimum volumetric capacity with the partitioning piston held at the spring-loaded position, and to prevent back flow to the pressure chamber when the piston is shifted towards the secondary chamber by a predetermined stroke and the fluid pressure in the pressure chamber becomes greater than the fluid pressure in the secondary chamber by a predetermined pressure level. With the previously-discussed arrangement of the prior art, when a master-cylinder pressure is produced with depression of the brake pedal, the master-cylinder pressure is transmitted from the pressure chamber to the secondary chamber, and then delivered through the pressure-differential sensitive external-pilot operating type directional control valve via the ABS unit (including a plurality of 2-port 2-position directional control valves) into the respective wheel-brake cylinders, thus ensuring a braking action depending on depression of the brake pedal. During the braking action, when a skid control for a less-traction skidding wheel is executed in response to a control command from an electronic control unit (ECU) or electronic control module (ECM), the return pump of the ABS unit is driven, a wheel-brake cylinder pressure of the less-traction skidding wheel is cyclically regulated in accordance with either one of three pressure control modes, namely a pressure-reduction mode, a pressure-hold mode (more precisely a high-pressure hold mode or a low-pressure hold mode), and a pressure build-up mode. These three modes can be repeatedly executed until the slip rate of the skidding wheel is adjusted within a predetermined target slip-rate range (containing an ideal slip rate). On the other hand, during so-called traction control (or acceleration-slip control) or during vehicle dynamics control (which includes at least a vehicle stability control, a steer characteristics control, and a cornering stability control), a braking force must be applied to a desired road wheel so as to prevent undesired acceleration slip by actively generating a braking force, or to change the steer characteristics to neutral steer, or to generate a yawing moment counteracting the undesired vehicle behavior about the z-axis, even when the brakes are released. In the previously-noted conventional braking system, during the acceleration-slip control or during the vehicle dynamics control, the ECU drives both the pressure apply pump and the return pump and also energizes electromagnetic solenoids of the 2-port 2-position directional control valves (inflow valves) to open the 2-port 2-position directional control valves. With the pressure apply pump activated, the brake-fluid pressure in the primary chamber of the pressure apply piston unit is increased to a high level, and thus the increased pressure forces the piston into the secondary chamber, thereby permitting the brake fluid in the secondary chamber to be pushed out via the inflow valves into the wheel-brake cylinders assigned to the pressure build-up mode during the traction-and-yaw control. During the traction control or during the vehicle dynamics control, a fluid pressure of the brake fluid, pushed out from the pressure apply piston and directed towards the wheel-brake cylinders, can be properly regulated by the ABS unit. On the other hand, since brake fluid is temporarily stored in a reservoir fluidly disposed in the return line when a pressure-reduction mode is being executed, just after the pressure-reduction mode the return pump can sufficiently suck brake fluid in from the reservoir of the return line and feed the pumped brake fluid to the wheel-brake cylinder. Thus, after the pressure-reduction mode, the pressure apply action of the pressure apply piston unit is unnecessary. The previously-noted pressure apply piston unit generally comprises a pressure-apply check valve. The check valve is designed to block fluid communication between the secondary chamber and the outlet/inlet port of the master cylinder when the pressure apply piston is axially shifted apart from the spring-loaded position to reduce the volumetric capacity of the secondary chamber. The previously-discussed conventional brake control system suffers from the following drawbacks.
The return pump and the pressure apply pump are driven by respective motors, and thus the prior art brake control system is large-sized. Supposing that a single motor is used for driving both the return pump and the pressure apply pump, even during the skid control (ABS control), an undesired pressure-apply action will be made by means of the pressure apply pump driven simultaneously during operation of the return pump and as a result the fluid pressure in the outlet port of the pressure apply pump becomes risen undesirably. This results in an increase in the motor load, and lowers an energy efficiency. Also, the conventional system is designed that the pressurized fluid from the pressure apply pump is supplied to a pair of brake-fluid pressure apply piston units each provided in the associated one of two channel brake systems and then the pressure is applied via the two pressure apply pistons to brake fluid in respective wheel-brake cylinders. It is desired that a braking action can be actively made to a desired wheel-brake cylinder of four wheel cylinders in response to a control command from the ECU even during the vehicle dynamics control with the brake pedal released. Actually, during the vehicle dynamics control, there is less possibility that active braking actions (or automatic, computer-controlled braking actions) for all of the four wheel cylinders are necessary. Usually, the number of wheel-brake cylinders, being subjected to each individual automatic braking action (a skid control or a traction-and-yaw control), is less than four. However, on the conventional system, during the vehicle dynamics control the pressure apply action would be made to all of the wheel-brake cylinders and thus the ABS unit must operate at either a pressure-reduction mode or a pressure-hold mode when a fluid pressure in a wheel-brake cylinder, which cylinder never requires the pressure apply action, is risen undesiredly. In other words, there is a tendency for the pressure apply pump and the pressure apply piston unit to be activated wastefully. Additionally, the conventional system is designed to shut off the fluid communication between the outlet/inlet port of the master cylinder and the ABS unit by means of only one check valve fluidly disposed in the secondary chamber of the pressure apply piston unit. If the fluid-flow passage area (the size of the valve opening) of this single pressure-apply check valve is narrowed due to debris, there is a tendency of constricting back flow from the wheel-brake cylinder to the master cylinder, thus disturbing satisfactory pressure-drop in the wheel cylinder when the driver releases the brakes.